former curator of the Imperial Valley transportation museum Steve wrote in the comments to inform us about this and said...

This was used for years to transport crews by rail over the narrow gauge railroad (last industrial narrow gauge railway in the state, if not the whole country) between the processing mill at Plaster City California, and the gypsum mine in the Fish Mountains 25 miles away.

With two front ends, it wasn't necessary to turn it around for the return trip. After it wore out it was acquired by the owner of the trucking company that hauled the finished sheetrock from the mill. About 12 years ago the vehicle was donated back to the company.

I can't reply by email to thank Steve because this blog set up doesn't require email addresses to comment. But I can thank you sincerely for providing me and all the readers with this incredible information that likely, you are the only person to know. It's astonishing that of all the people who might have known the information on the railcar, one reads my blog, and took the time to help us all know what this railcars history is.